The present invention relates to a device for measuring and processing an input signal of wide dynamic range, a leak detector and a corresponding measurement and processing method.
Electron multipliers are components which have an amplification function. Their gain is usually very high, possibly ranging up to 105. They are used in wide-ranging applications in which very weak signals are processed. Among other applications worth citing, there are the measurement of light energy (photomultipliers) in the visible or invisible, image intensifiers, the measurement of nuclear radiation (ionization chambers), mass spectrometry and in particular helium leak detection.
In these applications, the signals to be processed in certain cases exhibit a dynamic range of the order of 108. It is therefore necessary to have a measurement system which comprises the electron multiplier and a signal compressor amplifier. This element may not, on its own, adequately reduce the output dynamic range. It is then necessary to act on the gain of the electron multiplier to implement an additional compression. This is obtained by acting on its power supply voltage. In known devices, the gain of the measurement system, notably by action on said power supply voltage, is controlled discontinuously, either manually by the use of a switch or, in more sophisticated systems, by automatic electronic switching or by multiple measurement systems. This discontinuity in the transfer functions generates undesirable effects such as significant response times or instabilities when the signal is at the limit of two amplification ranges.
Already known from the document EP 0 402 827 is a device for processing the signal received by an electron multiplier, allowing for a continuous measurement of the signal over a wide dynamic range, comprising an electron multiplier receiving the input signal Io, a high-voltage power supply for the multiplier provided with a control circuit, a feedback loop including a compressor amplifier delivering the output signal and also acting on the control circuit so as to vary the gain of the electron multiplier continuously over the measurement dynamic range, as a function of the output signal IoG of the electron multiplier.
The device implemented in this way makes it possible to process an input signal with high dynamic range to obtain a usable linear output signal that can represent the input signal received by the electron multiplier over a wide band of the input signal extending up to eight decades.
The high voltage power supply determines the gain of the electron multiplier. This power supply is modulated by the control circuit receiving the output signal of the device. For this, the control circuit has adjustable gain and offset parameters which define the band of the output signal.
To adjust these gain and offset parameters, the conventional method consists in injecting two different input currents and in adjusting the corresponding gain and offset parameters for each value of the input current.
However, this adjustment does not take immediate effect. In practice, the gain and offset parameters of the control circuit are interdependent and mutually influential. In other words, the adjustment of the gain parameter of the control circuit leads to the modification of both the offset and of the gain of the band of the output signal. Similarly, the successive adjustment of the offset parameter of the control circuit leads to the modification of both the offset and the gain of the band of the output signal. The interdependence of these two parameters demands a fine adjustment of the parameters of the control circuit, requiring a certain number of iterations to achieve the desired output signal band. This adjustment may take a relatively long time, given that several minutes may be needed to obtain a satisfactory stabilization of the output signal IoG after each modification of the injected input current Io.